I just got 4 transistor amps previous used on aeronautical frequencies.It is a triple stage amp with 10 or 100 mW drive for full output.The amps have 50w transistor on the final stage and do probably run in class C at 24V on the two final stages in the amp and probably 12V on the first stage.Can anybody give me some advise about how to tune them for 70 Mhz.I guess a start would be to bypass the output filter and add a variable cap in the output matching network of the transistor.

For testing I'll probably use an attunator to keep the output on a low level.

I'm a bit uncertain on how to change the class C to a class AB and get the bias correct.

Which aeronautical frequencies? There are aero frequencies at 75 MHz ( ILS outer beacons around 315 MHz, communications /VOR at 108 to 136 - Upper end depends where you are.What frequencies are you referring to?

Quite a job then. You'll need to redesign the matching networks for the lower band: start with the transistor data - it is possible, but unlikely (depending on age) that the transistors are internally matched for the 108 - 136 MHz band, in which case it really is a job, and in any case, you may well find it easier to start from scratch or follow a published design.

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